1. Field of the Invention
The present invention relates to methods for the synthesis of condensation polyimides that contain a 1,1,1-triaryl-2,2,2-trifluoroethane structure, as well as other related condensation polyimides. The said condensation polyimides meet or exceed the performance characteristics required for high temperature resins and composites in present or future aeronautic requirements.
The utility of polyimides as a class of polymers is well-known. Polyimides, because of their low cost, excellent thermo-oxidative stability, chemical stability, and commercial availability, are in a class by themselves. They exhibit widespread applications, such as for films, coatings, moldings, adhesives, binder solutions and matrix resins. Extensive review articles and books, such as in Heat-Resistant Polymers, J. P. Critchley, et al, chapter 5, on Polyimides, (Plenum Press, 1983) described the many polyimides that have been prepared and indicate which compositions are successful commercial variants. In a class by themselves are the propylidene (6F) polyimides because of their superior thermal stability compared to non-fluorinated polyimides. The 6F containing polyimides have become the state-of-the-art in melt processable polyimide for moldings and matrix resin applications. This patent application is designed to show that the use of the new 1,1,1-triaryl-2,2,2-trifluoroethane (3F) containing polyimides possess all the desirable melt fusible-melt processable characteristics of 6F containing polyimides but in addition are more versatile than 6F containing polyimides because of the potentials described to introduce a variety of functional groups for the modification of polymer properties. In addition, the 6F containing polyimides have a very unique new application compared to non-6F polyimides. The use of 6F containing polyimides as film formers for protective coatings on solar cells was recently patented (U.S. Pat. No. 4,592,925 granted to Preston S. duPont and Normn Bilow of Hughes Aircraft Company) because the use of 6F connecting groups in polyimides was shown to provide almost transparent polyimide films, as reported in A. K. St.Clair, et al, NASA TM 87650 (December 1985). Because it was also noted in the Alston and Gratz work on 3F containing polyimides that the films were very light in color compared to the non-6F polyimides, then the 3F polyimides would also have applications as film forming resins for application where optical transparency is a desirable characteristic, e.g., solar cell coatings.
2. Description of the Prior Art
Presently one method of meeting these high temperature performance requirements has been through the use of commercially available thermo-oxidatively stable monomers/polymers based on a 2,2-diaryl-1,1,1,3,3,3-hexafluoro-isopropylidene structure (6F). A 1,1,1-triaryl-2,2,2-trifluoro-ethane structure (3F) is also known. ##STR1##
The synthesis of condensation polyimides based on similar 6F structures has been previously reported as follows: H. G. Gibbs, J. Applied Polymer Science, Applied Polymer Symposium, 35, 207 (1979); E. I. duPont, British Pat. No. 1,062,435 (Mar. 22, 1967). ##STR2##
The 6F dianhydride is presently used in commercially available resin products (called NR 150 resins) available from E. I. duPont, U.S. Pat. No. 3,310,573. The 6F diamine polymerized with pyromellitic dianhydride has been patented but not commercialized by E. I. duPont. A similar 6F diamine wih meta, rather than para substitution, has been recently synthesized, K. S. Y., Lau, et al, J. Polymer Science, 20, 2381-2393 (1982), and is available from American Hoechst in developmental quantities but reference does not exist to its use in polyimides. Nevertheless, during the course of this study, a new polymer of pyromellitic dianhydride/meta 6F diamine was prepared for comparison purposes to 3F containing polymers as shown below. ##STR3##
The synthesis of similar diamines but based on a 3F structure through a different synthetic route has been reported, W. D. Kray and R. W. Rosser, J. Org. Chem., 42, 1186-1189 (1977). The purpose of this application is to disclose (or claim ) new 3F containing high temperature resistant condensation polyimides and to demonstrate two methods of controlling the high glass transition temperature (Tg) of 3F diamine containing polymers. This is reported in FIGS. 6 and 7 of William B. Alston and Roy F. Gratz, NASA TM 87113, AVSCOM TR-85-C18. These polyimide polymers contain the 3F structure in both the diamine or dianhydride monomers. The 3F diamine synthesis is reported in W. D. Kray and R. W. Rosser, J. Org. Chem., 42, 1186-1189 (1977) while the 3F dianhydride synthesis is disclosed in U.S. Pat. No. 4,758,380.
This copending disclosure claims a variety of methods to effect additional functional group introduction, such as from during the initial monomer synthesis to during a later step in the monomer synthesis, to performing functional group changes on the final polymer structure. This U.S. Pat. No. 4,758,380 disclosure generally claims the polymerization to new 3F polyimides after the functional groups for modifying polymer properties are already within the monomer structures, but is not limited to introduction of functional groups prior to polymerization. Changes in functional groups may be effected once a functional group already exists within the 3F polymer. Thus, the utility of the 3F group is primarily because the 3F phenyl ring is a synthetic site which represents these 3F polymers as the next generation of more synthetically versatile polymers than with the state-of-the-art 6F polymers currently claimed in U.S. Pat. No. 4,592,925 as solar cell coatings or also as radiation resistant or radiation sensitive modified 6F polymers as patented in U.S. Pat. No. 4,416,973 by David L. Goff of E. I. duPont. An example is the use of substituted anilines to prepare alkyl substituted 3F containing diamines as outlined in U.S. Pat. No. 4,758,380 has expanded utility as potential photoresists advancing the technology of non-fluorinated photoresists described by J. Pfeifer and O. Rhode of Ciba-Geigy in "Direct Photoimaging of Fully Imidized Solvent Soluble Polyimides" in proceedings of 2nd International Conference on Polyimides, 10/30/85-11/1/85 at Ellenville, N.Y.